The present invention relates to a method for fabricating optoelectronic devices, such as lasers, modulators, optical amplifiers, and detectors, and in particular to a method and device for reducing the diffusion and/or interdiffusion of dopant atoms among differently doped regions of such optoelectronic devices.
Blocking layers are increasingly important for optoelectronic devices. For example, in a buried heterostructure of a semiconductor laser diode, blocking layers confer superior characteristics, such as low oscillatory threshold value and stable oscillation transverse mode, as well as high quantum efficiency and high characteristic temperature. This is because, in the buried heterostructure laser diodes, a current blocking layer can be formed on both sides of an active laver formed between two clad layers having a large energy gap and a small refractive index. This way, current leakage during operation is substantially reduced, if not prevented.
A conventional method for the fabrication of semiconductor laser diodes having a semi-insulating buried ridge is exemplified in FIGS. 1-7 and described below.
Referring to FIG. 1, the processing steps for fabricating a laser diode with a buried ridge begin with the formation of a multi layered structure 100 on an n-InP substrate 10. The multi layered structure 100 is formed of a first n-InP cladding layer 12, an active layer 14, a second p-InP cladding layer 16, and a layer 18 of a quaternary material (Q). Layers 12, 14, 16 and 18 are sequentially formed and successively epitaxially grown to complete a first crystal growth. The active layer 14 could be, for example, a multiple quantum well (MQW) structure formed of undoped InGaAs/InGaAsP pairs and formed by a Metal Organic Chemical Vapor Deposition (MOCVD) or Metal Organic Vapor Phase Epitaxy (MOVPE). Also, the second cladding layer 16 may be doped with a p-type dopant, the most common one being zinc (Zn).
Next, as shown in FIG. 2, a SiO2 or Si3N4 mask 20 is formed unto a stripe on the upper surface of layer 18. Subsequently, the multi layered structure 100 is selectively etched down to the n-InP substrate 10 to produce a mesa stripe 50, as illustrated in FIG. 3. The mesa stripe 50, which has the mask 20 on top, is then introduced into a growth system, such as a liquid phase epitaxial, a MOCVD, a molecular beam epitaxy (MBE), or vapor phase epitaxy (VPE) growth system, so that an InP current blocking layer 32 and an n-InP current blocking layer 34 are subsequently formed, as shown in FIG. 4. The current blocking layers 32 and 34 surround the mesa stripe 50 and form a second crystal growth.
The first current blocking layer 32 may be doped with impurity ions, such as iron (Fe), ruthenium (Ru) or titanium (Ti), to form a semi-insulating (si) InP(Fe) blocking layer 32. The addition of Fe-impurity ions increases the resistivity of the first current blocking layer 32 and reduces the leakage current that typically occurs at the interface between the substrate 10 and the first current blocking layer 32. Similarly, the second current blocking layer 34 may be doped with impurity ions, such as silicon (Si), sulfur (S) or tin (Sn), to form an n-type InP-doped blocking layer 34.
Referring now to FIG. 5, after removal of the mask 20, a third crystal growth is performed on the upper surfaces of the second current blocking layer 34 and the Q layer 18. Thus, a p-InP cladding layer 42 (also called a burying layer) and a p-InGaAsP or a p-InGaAs ohmic contact layer 44 are further grown to form a buried heterostructure. The cladding layer 42 may be also doped with p-type impurity ions, such as zinc (Zn), magnesium (Mg), or beryllium (Be), to form a p-type InP-doped cladding layer 42. Since Zn is the most commonly used p-type dopant, the cladding layer 42 will be referred to as layer InP(Zn)-doped.
The method of fabricating the above structure poses three major drawbacks, all of them relating to the diffusion and interdiffusion of dopant atoms, particularly those of zinc, since zinc is the most common and widely used p-type dopant in the optoelectronic industry.
First, zinc diffusion occurs into the active region of the semi-insulating buried ridge. FIG. 5 shows the diffusion of zinc in the direction of arrow A, from the doped p-InP(Zn) second cladding layer 16 into the active layer 14, because of the direct contact between the two layers. The high diffusivity of zinc leads to an undesirable shift in the emitting wavelength, up to tenths of microns. The reshaping of the overall zinc distribution profile further impacts the electrical characteristics of the optoelectronic device. The excess of zinc in the active region 14 of the device structure also results in the degradation of various device characteristics, such as the extinction ratio and the junction capacitance of the electro-absorption modulator strictures.
Second, iron-zinc (Fexe2x80x94Zn) interdiffusion occurs at the interface between the doped p-InP(Zn) second cladding layer 16 and the semi-insulating InP(Fe) first current blocking layer 32. FIG. 5 shows the diffusion of zinc in the direction of arrow B1, from the p-InP(Zn) second cladding layer 16 into the InP(Fe) first current blocking layer 32. Similarly, arrow B2 of FIG. 5 illustrates the diffusion of iron from the InP(Fe) first current blocking layer 32 into the p-InP(Zn) second cladding layer 16.
Third, iron-zinc (Fexe2x80x94Zn) interdiffusion occurs in the blocking structures of the laser devices, more precisely at the interface between the semi-insulating InP (Fe) first current blocking layer 32 and the p-InP(Zn) cladding layer 42. The problem arises because the Fe-doped InP current blocking layer 32, which was initially covered by the mask 20, comes into contact with the Zn-doped InP cladding layer 42 after the removal of the mask 20. The contact regions are exemplified in FIGS. 5 as regions D, situated on lateral sides of the mesa stripe 50. The interdiffusion of Fe and Zn atoms at the regions D can significantly increase the leakage current and degrade the device, leading to a poor manufacturing yield. In addition, if the active layer 14 has a multiple quantum well (MQW) structure, the Zn impurities in the Zn-doped InP cladding layer 42 can enter the active layer 14 to form mixed crystals therein and practically reduce the quantum effect to zero.
In an effort to suppress the diffusion and interdiffusion of Zn dopant atoms, different techniques have been introduced in the IC fabrication. For example, one technique of the prior art, shown in FIG. 6, considered the incorporation of a zinc doping set-back into the device structure, such as an undoped InP layer 52. The undoped InP layer 52 is grown after the growth of tie active layer 14, but before the growth of the p-InP second cladding layer 16, to prevent therefore the direct contact between zinc and the active region. In lieu of the undoped InP layer 52, a silicon doped n-InP(Si) layer may be used also as a dopant set-back.
Although the above technique has good results in preventing tie Zn diffusion, its processing steps require extremely sensitive parameters, such as doping level and thickness, of the zinc-doped cladding and contact layers. Also, growth conditions, such as growth rate and temperature, must be very narrowly tailored so that the set-back is optimized for each device structure and for each reactor. Further, this method does not allow control over the shape of the final zinc distribution. Finally, when a silicon doped n-InP(Si) layer is alternatively used as a dopant set-back, the incorporated silicon, which is an n-type dopant, forms an additional and undesirable p-n junction on the p-side of the device.
Another technique of the prior art that tried to minimize the zinc-iron interdiffusion is exemplified in FIG. 7. This technique contemplates the insertion of an intrinsic or undoped InP layer 70 between the Fe-doped InP current blocking layer 32 and the Zn-doped InP cladding layer 42, to prevent the contact between the InP(Fe) layer and InP(Zn) layer and to eliminate the iron-zinc interdiffusion and the consequent leakage current. This technique, however, has a major drawback in that it affects the p-n junction between the n-InP second current blocking layer 34 and the p-InP burying layer 42. Specifically, the addition of an intrinsic InP layer modifies the p-n junction that should be in the active region of a laser device, and creates instead a p-i-n junction that alters the device characteristics altogether. Further, this method is insufficient to completely prevent the iron-zinc interdiffusion in areas close to the active region of the device.
Accordingly, a method for forming a mesa stripe for optoelectronic devices, which is inexpensive to implement and capable of decreasing the leakage current and the interdiffusion of dopant atoms is needed. There is also a need for an optoelectronic semiconductor device having good operating characteristics with reduced impurity atoms interdiffusion, reduced leakage current, and improved accuracy and operation reliability.
The present invention provides a method for reducing the diffusion and/or interdiffusion of dopant atoms between differently doped regions of semi-insulating buried ridge structures of forward biased devices, such as lasers and optical amplifiers, and of reverse biased devices, such as electroabsorption modulators and detectors.
In a first embodiment of the present invention, either an InAlAs (indium aluminum arsenide) or an InGaAlAs (indium gallium aluminum arsenide) layer is grown on top of the active region, and before the zinc-doped cladding layer and the subsequent contact layer are grown. The blocking of zinc diffusion into the active layer by the insertion of a thin InAlAs or InGaAlAs layer allows a precise placement of the p-i junction, at less than 100 Angstroms, as well as minimal doping into the active region.
In a second embodiment of the present invention, an InAlAs or an InGaAlAs layer is first selectively grown on top of the active region and around the mesa structure, and only then are conventional InP and n-InP current blocking layers, which form a second crystal growth around the mesa, grown over the InAlAs or InGaAlAs layer. This way, the lateral interdiffusion between Fe atoms, from the InP(Fe) current blocking layer, and the Zn atoms, from the p-InP(Zn) second cladding layer situated on top of the active region, is suppressed since no contact between the two doped regions exists.
In yet a third embodiment of the invention, a plurality of InAlAs an and/or InGaAlAs layers are grown on top of the active region and around the mesa structure, as well as in lieu of the conventional second current blocking layer of the second crystal growth.
A fourth embodiment of the present invention is structurally similar to the third embodiment. However, in the fourth embodiment, the InP(Fe) current blocking layer is grown between two adjacent InAlAs and/or InGaAlAs layers, so that the InP(Fe) layer has minimal contact with the mask situated on top of the mesa stripe.
According to fifth and sixth embodiments of the present invention, a plurality of InAlAs and/or InGaAlAs layers are grown on top of the active region and around the mesa structure, as well as in between the blocking layers forming the second crystal growth. In the fifth embodiment, an InAlAs and/or InGaAlAs layer is grown after the two current blocking layers have been formed and as part of the second crystal growth. Conversely, in the sixth embodiment, an InAlAs and/or InGaAlAs layer is grown as part of the third crystal growth and before the top cladding layer is formed. In any case, these multiple InAlAs and/or InGaAlAs layers suppress the interdiffusion between Fe atoms, from the InP(Fe) current blocking layer, and the Zn atoms, from the p-InP(Zn) cladding layer of the third crystal growth. Multiple InAlAs and/or InGaAlAs layers may be incorporated in the blocking and/or cladding structures to confer optimized performance to the optoelectronic devices.